Andalusite is a polymorph of kyanite and sillimanite. These three minerals share the same formula, but are stable at different P/T conditions.
Physical Properties |
|
Chemical formula | As2SiO5 |
Class | Nesosilicate Isolated tetrahedra |
Crystal system | Orthorhombic |
Habit | Stubby to elongate square prisms Individual crystals may be rounded Massive Granular |
Color | Brown to red |
Hardness | 7.5 |
Specific gravity | 3.18 |
Cleavage | Rarely seen, good {110}, poor (100) |
Fracture | Subconchoidal |
Luster | Vitreous |
Transparency | Transparent to translucent |
Streak | White |
Optical Properties |
|
PPL | Usually colorless May be weakly colored and pleochroic |
XPL | 1st order yellows (pale tan) May be euhedral with a square outline and penetration twins or cross pattern |
δ | 0.010 |
Twinning | Penetration twins or Maltese cross pattern |
after Perkins, 345 |
Andalusite in Hand Sample
Four small subhedral to euhedral andalusite crystals
Andalusite in a rock
Andalusite in Thin Section
Thin Section GigaPans
Andalusite in slate, plane polars
Andalusite in slate, crossed polars

Andalusite, Plane Polars
Andalusite in plane polarized light. Magnification: 40x Rock: Slate Click here to watch this grain rotate in crossed polars: https://youtu.be/_choy6NVlzc Cli…

Andalusite, Crossed Polars
Andalusite in cross polarized light. Magnification: 40x Rock: Slate Click here to watch this grain rotate in plane polars: https://youtu.be/o3eT2k9daRM Click…
Further Reading
Andalusite at webmineral.com
Andalusite at mindat.org
Bonus Content
The song that gets stuck in your friendly author’s head every time she encounters andalusite.
The Clash Spanish Bombs
The Clash – Spanish Bombs With Lycris Spanish songs in Andalucia the shooting sites in the days of ’39 Oh, please leave, the ventana open Federico Lorca is d…